The present invention relates to a calibration system for a combination weighing apparatus, and more particularly relates to a calibration system adapted to calibrate a zero point of weighing buckets included in a combination weighing apparatus wherein the weighing buckets are weighed at their empty state but during their normal operation period in order to improve accuracy and efficiency in combination processing.
There is known a calibration system in a combination weighing apparatus which comprises a plurality of sets of buckets, each set being composed of one feeding bucket "FB" and one weighing bucket "WB" as is shown in FIG. 16 wherein contents of the weighing buckets are directly discharged into a collecting chute. The known calibration system in such a basic set of buckets is illustrated on a time chart shown in FIG. 17. The time chart shows calibration of zero point of the weighing buckets included the basic set of buckets on the so-called "double-shift operation" which is composed of a first shift or mode of weighing sequence and a second shift or mode of weighing sequence. The reference symbol "C" on the horizontal bars in FIG. 17 denotes a period of time needed to calculate possible combinations, each combination including weighed masses of material or the weighing buckets "WB" containing same. The masses are of given weights and are to be subject to selection. The other reference symbol "D" denotes a period of time required to discharge the mass in selected bucket into the collecting chute. The further reference symbol "W" indicates a further period of time for the "actual" weighing by said weighing bucket "WB", the "actual" weighing being done to determine the weight of said mass. The still further reference symbol "Z" indicates a still further period of time for the "blank" weighing of an empty weighing bucket, the "blank" weighing being done to calibrate the zero point of said weighing bucket.
A group "Sa" of the horizontal bars in FIG. 17 indicate a case wherein no calibration of zero point is conducted during the second shift of weighing sequence. The weighing of a succeeding mass begins as shown by "W" immediately after a preceding mass has been discharged as shown by "D", in this mode. Another group "Sb" of the horizontal bars in FIG. 17 indicate another case wherein calibration of zero point is conducted at regular intervals of time or every time when a predetermined number of masses have been discharged from the weighing bucket "WB", during the second shift of weighing sequence. In the example illustrated in FIG. 17, the feeding bucket "FB" does not supply the weighing bucket "WB" with the succeeding mass after the latter bucket has discharged an n-th mass as shown by the symbol "D". Thus, "blank" weighing of said weighing bucket "WB" is carried out as shown by "Z" immediately after finish of the discharge. A weight signal obtained by such a blank weighing will be transmitted as a tare weight of said weighing bucket to a control system described later, after the intensity of said signal has become stable.
The "actual" weighing "W" will be reopened by the weighing bucket "WB" after the "blank" weighing "Z" has completed, receiving the next mass from the feeding bucket "FB". It is, however, impossible for the weighing bucket to finish the reopened "actual" weighing of the succeeding mass before the calculation of combination for (n+2)th discharge cycle starts. Thus, the next mass cannot be made subject to said calculation for the (n+2)th discharge. In other words, such a considerably long period of time is necessary for the "blank" weighing "Z" due to the period needed to stabilize the signal of tare weight which is utilized to accurately calibrate the zero point of the weighing bucket "WB". Therefore, the "blank" weighing during a normal operation period of the weighing bucket "WB" will make it impossible to obtain a well stabilized weight signal of the next mass before the calculation of combination of masses for the next discharge will start.
It will be apparent that the described known system for calibration of zero point is disadvantageous because the calibration is conducted at regular time intervals or for every predetermined number of discharges. This reduces the number of weighing buckets available to the selection in the combination processing, thereby affecting the accuracy and efficiency in combination of the masses contained in said weighing buckets. In the combination weighing apparatus which comprises the basic sets of buckets, the effective number of available buckets depends upon the number of weighing buckets "WB" from which the preceding masses have been discharged in the aforementioned "double-shift" operation.
An example of the known apparatuses is now described which is composed of fourteen (14) sets of buckets, each set comprising one feeding bucket "FB" and one weighing bucket "WB", in order to more fully describe the effective number of the available buckets.
In an assumptive case wherein four (4) weighing buckets "WB" have discharged their contents at (n+1)th discharge cycle, the number of available weighing buckets will be ten (10) (=14-4) for the next (n+2)th discharge cycle. It is possible that three (3), four (4) or five (5) weighing buckets "WB" have to discharge the weighed masses contained therein even if an average number of available weighing buckets "WB" were set to be four (4) for each discharge cycle. This variation is caused by unevenness in the weight of masses in the weighing buckets. Thus, probabilities are about 20%, 60% and 20% respectively for the cases of discharge from three, from four and from five weighing buckets.
The above-mentioned assumptive case wherein ten (10) buckets are available for the (n+2)th discharge provides two hundred and ten (210) (=10.sup.C 4) combinations of the weighing buckets "WB". The number of such combinations will be reduced to one hundred and twenty-six (126) if five (5) weighing buckets have discharged at the preceding (n+1)th cycle so that the number of available buckets is nine (9) for the (n+2)th cycle. Said number of combinations will be further decreased to, for instance, seventy(70) (=8.sup.C 4) if one bucket undergoes zero-point calibration to be inactive at the next cycle. Such a decrease in the number of available weighing buckets will seriously lower the accuracy of combination.
It is to be noted that the zero-point calibration is carried out to compensate any drift in zero-points of sensors or amplifiers, which drift may be caused by fluctuation of ambient temperature or by adhesion of the weighed material to walls of the weighing buckets "WB". Generally speaking, such zero-point calibration is effected for instance at intervals of ten (10) minutes or for every thousand (1000) discharges. Therefore, said calibration is not a matter of urgency or emergency.